The present invention relates to a novel interface device, which permits a universal input/output to personal computers (hereinafter simply referred to as "PCs"), by using a printer port which is found on most PCs.
In general, the printer port of the PC is composed of DATA lines for sending out characters or graphic data to be printed by a printer, a STB line for sending out a synchronizing output signal allowing data to be read in the printer, that is, a STB (STROBE) (hereinafter abbreviated) signal, a READY line for receiving a signal informing whether or not data can be read in from the printer, that is, a READY (or BUSY) signal, and an ACK line for receiving a data required pulse from the printer, that is, an ACK (ACKNOWLEDGE) (hereinafter abbreviated) signal (hereinafter the upper bar " " is omitted).
The timing chart of the DATA signal at the printer port in the PC and the control signals which are composed of STB, READY and ACK signals is as shown in FIG. 1. In the PC, first of all, a datum which is necessary for the DATA line is sent out, and then, a STB pulse requiring data to be read is developed as an output. The printer which has received the STB pulse makes the level of the READY signal high (H) so as to read the data (shown by the dotted lines (a) and (a') in FIG. 1). When the operations, for example, of reading the data or printing comes to an end, the level of the READY signal is made low (L) so as to supply the pulse (ACK), which requires data, as an input to the PC. Otherwise, the pulse (ACK) is supplied as an input to the PC as shown by the dotted line, so that the level of the READY signal is made low (L) in response to the rise thereof. The PC which has received the same prepares the next DATA (shown by the dotted line (b) in FIG. 1), and the STB signal is again developed as an output.
The sending and receiving of signals as stated above is relatively well standardized, and similar signal lines are used in PCs of different manufacturers. Therefore, the interface means, to which are connected machines and instruments via the printer port, is generally applicable to all such PCs.
However, since the main object of the printer port is that of transferring the printing data, the input to the PC is limited to the READY and ACK signals and the like even if the data are parallel data, and the role thereof is so determined. Therefore, if the printer port is used as an input/output port, the input information line to the PC would not be sufficient.